Driving a display device, such as a computer monitor or a liquid crystal display (LCD), accounts for a substantial percentage of the power consumed in operating a computer system. Thus, a major source of energy inefficiency in computer operation is the requirement to drive a display device during periods of inactivity. The energy inefficiency is particularly disadvantageous in circumstances in which power is supplied by a battery, e.g. a laptop computer.
Circuits for blanking a display screen during periods of inactivity are known. Screen blanking conserves power and extends the charge life of a battery. Moreover, screen blanking prevents image "burn-in," i.e. screen phosphor deterioration that leaves a permanent ghost image if the same image is left on a screen for an extended period of time.
A simple screen saving circuit is one in which a single timer is connected to monitor activity by user-interface devices and by video random access memory (VRAM). In the absence of any activity, the timer issues an interrupt that disables power to a display device. "User-interface devices" is defined herein as devices used by an operator of a computer system to input data or commands. User-interface devices include a keyboard, a mouse and a touch-screen display device. Activity by the user-interface device or activity at the output of the VRAM will reset the timer and, if the interrupt has been issued, will disable the interrupt to return power to the display device.
A modification of the circuit is to provide separate timers for monitoring VRAM activity and activity by the user-interface device. The timers can be set to measure different periods of time, with each timer being connected to initiate generation of an interrupt. A double-timer circuit requires additional hardware logic and a more complicated software implementation, but resolution is enhanced.
A third screen blanking circuit is described in U.S. Pat. No. 5,059,961 to Cheng. The screen blanking circuit of Cheng has three inputs. A first input is connected to the VRAM of a computer system. The second and third inputs receive the horizontal and vertical synchronizing signals from a cathode ray tube (CRT) controller of a display device to be selectively disabled. If during a selected time period no read/write signal is received from the VRAM along the first input, the horizontal and vertical synchronizing signals are electrically disconnected from the display device, causing the data image to disappear.
One problem with the Cheng circuit is that electrically disconnecting the horizontal and vertical synchronizing signals to the display device does not turn the display device "off." While power consumption is reduced, the cathode ray tube itself remains "on." The tube beam will not sweep, but instead will be substantially fixed. Another problem is that the Cheng circuit is limited to use with cathode ray tubes. Portable computer systems typically use LCD devices and other means which do not require horizontal and vertical synchronizing signals.
One difficulty of each of the screen blanket circuits described above is that monitoring video circuitry, such as the VRAM, will potentially defeat the purpose of the circuit. For example, if the display includes a digital clock, a video update will occur every second. If the VRAM timer is set to measure a period greater than one second, the screen will remain on continuously. On the other hand, if the time is set for less than one second, the screen will turn off, but will turn back on at the one-second update. The frequent on/off fluctuation will not result in a savings of power and will likely shorten the use-life of the display device. At the very least, the frequent switching will be a source of distraction to the user. Similar problems are encountered during protracted "number crunching" performed by a spreadsheet program. During the time necessary to reach a file tally, the video may be updated periodically with intermediate results. Each update will cause a screen blanker to turn the screen back on, even though the user is likely to have no interest in the intermediate results.
An object of the present invention is to provide an apparatus and method for controlling video display in an efficient manner without encountering problems caused by managing video updating circuitry.